The invention relates to improvement in signalling protocol between a base station transmitter site and a remote dispatch terminal. In particular, the invention relates to an improvement in the tone remote control of signalling scheme described in U.S. Pat. No. 3,577,080 to Cannalte. The invention is related to U.S. Pat. No. 4,455,617 issued 6/19/84 and U.S. Pat. No. 4,433,256 issued 2/21/84.
In Cannalte when a remote dispatch terminal wants to key its associated base station transmitter, it generates a two tone sequence consisting of high level "guard" tone followed by a "function" tone. These tones are decoded at the base station causing the base station to respond by carrying out some particular task. Some of the "function" tones instruct the base station to key the transmitter. Usually a voice audio message from the remote dispatch terminal follows a tone which keys the transmitter. The signalling scheme described in U.S. Pat. No 3,577,080 to Cannalte has both a high level and a low level "guard" tone signal. Both "guard" tones are of the same frequency. A low level "guard" tone is sent simultaneously with a voice audio message. This low level "guard" tone is decoded by the base station and used to determine the duration of the voice audio. When the voice audio is complete, both the voice and the lowlevel "guard" tone are discontinued. When the detector at the base station senses the absence of the low level "guard" tone, the transmitter is deactivated or dekeyed and thereby made ready for the next command from the remote dispatch terminal. Since the "guard" tone is a single frequency, the decoder for the tone consists primarily of a high quality factor filter. Because of the high quality factor the decoder reacts relatively slowly when the "guard" tone frequency is discontinued. Due to the high quality factor characteristics of the low level "guard" tone decoder, approximately 100 to 500 milliseconds of silence are required at the end of a transmission from the remote dispatch terminal before a new message transmission begins. This dead time or silence is necessary in order to insure the high quality factor decoder has sufficient time to respond to the discontinuation of the low level "guard" tone.
Under noisy input line conditions, the ordinary 100 to 500 milliseconds of silence at the end of a transmission from the remote dispatch terminal has been known to increase substantially since the low level "guard" tone detector mistakes line noise as a valid signal. The low level "guard" tone is of relatively low amplitude and as such the associated decoder at the base station transmitter site is particularly sensitive to noise. Accordingly, a noisy input line together with the high quality factor nature of the "guard" tone decoder often causes the decoder to continue indicating to the base station circuitry that a low level "guard" tone is present after the remote dispatch terminal has discontinued the low level "guard" tone. The result is that the transmitter remains activated after a message transmission has completed. The base station circuitry will not allow the transmitter to dekey and return the base station to a condition that is ready to receive a new message transmission. Since the low level and high level "guard" tones are at the same audio frequency, the base station transmitter decoder is unable to distinguish between the two signals. The base station can only discriminate between high and low level "guard" tone by the tones position in the message transmission. Therefore, if the base station transmitter hangs up under a noisy input line condition, the base station ignores any high level guard tone sent by the remote dispatch terminal since the state of the base station circuitry is such that it senses the high level "guard" tone signal only as a strong low level "guard" tone.
Currently, in the Cannalte signaling scheme the remote dispatch terminal must wait 500 milliseconds between messages in order to insure that a second message will be properly received. When line conditions are particularly noisy, even this 500 millisecond wait is sometimes not long enough. Base station transmitters have been known to hang up indefinitely under high input noise conditions requiring service technicians to actually visit the base station site in order to dekey the transmitter. In addition, the silent period required between message transmissions is dead air time that slows down the system throughput and is especially burdensome in high speed data systems. Also, when there is more than one remote dispatch terminal for a base station transmitter (parallel consoles), there is no inherent mechanism for a supervisor station to override and take over control of the transmitter by sending a command to dekey the station.
Therefore, the object of the invention is to provide a mechanism to sense the presence of high level "guard" tone under condition in which the signalling scheme calls for the base station to be detecting low level "guard" tone.
It is a further object of this invention to provide a way to dekey the transmitter when the base station appears to be hung up either by line noise or by parallel console failure or misuse.
It is another object of this invention to totally eliminate the silent time following each transmission from a remote dispatch terminal so that a series of messages can be sent immediately following one another.